The present invention relates to a plasma CVD process for forming a metal film used for a step of fabricating a semiconductor device, a process for forming a metal nitride film used for a step for fabricating a semiconductor device, and a semiconductor device using a metal film or metal nitride film formed by the above process. In particular, the present invention relates to a plasma CVD process for a metal film and a formation process for a metal nitride film which are intended to reduce the amount of a halogen element remaining in a film formed, to improve a homology of a film, and to improve the symmetry of the coverage of a film, and a semiconductor device using a film formed by the above process.
The design rule of semiconductor devices such as LSIs is being made finer from a quarter-micron level to a sub-quarter micron level and also a multi-layered interconnection structure is being extensively used. With such a tendency, the aspect ratio of a contact hole for connecting an upper layer interconnection to a lower interconnection becomes larger. For example, in a semiconductor device having a design rule of 0.18 .mu.m, the opening diameter of a contact hole is 0.2 .mu.m and the thickness of an interlayer insulating film is in a range of about 1.0 to 2.0 .mu.m, and consequently the aspect ratio is in a range of 5 to 10. To achieve a multi-layered interconnection structure with a high reliability through such a contact hole having a fine diameter and a high aspect ratio, there is generally adopted a method of forming in the contact hole a metal film made from Ti or the like for ohmic contact and a thin metal nitride film made from TiN or the like as a barrier metal for preventing diffusion of an interconnection material, and forming a contact plug or an upper interconnection layer by high temperature sputtering of an Al based metal or selective CVD or blanket CVD of tungsten (W), thereby filling the contact hole therewith.
In general, a Ti film or a TiN film is formed by sputtering or reactive sputtering using a bulk Ti metal as a target metal. Such a sputtering process, however, fails to solve a problem on step coverage. To solve the problem, there is expected a method of forming a conformal Ti film or TiN film by CVD using chemical reaction on the surface of the substrate to be processed.
The CVD processes for forming a film made from a Ti based material, which have been proposed at the present time, is mainly classified into two types. One type uses an organic metal compound such as TDMAT or TDEAT, as reported, for example, in Proc. 11th. Int. IEEE VMIC, pp. 440 (1994). The other type uses an inorganic metal halide such as TiCl.sub.4, as reported, for example, in the Proceedings of the 44th Symposium of Institute of Semiconducto r/Integrated Circuit Technique, pp. 31 (1993).
In the latter type thermal CVD process for forming a Ti film, reduction between TiCl.sub.4 as a metal halide and molecules of H.sub.2 is an endothermic reaction expressed by the following chemical equation (1). This reaction slowly proceeds from the viewpoint of thermodynamics (.DELTA.G in the equation indicates a standard heat of formation). The shape of the film formed in this thermal CVD is conformal. To be more specific, the film is uniformly deposited on both a bottom surface and a side wall of the contact hole, so that the opening width of the contact hole is made narrow. This makes it difficult to bury the contact hole with tungsten or the like. EQU TiCl.sub.4 +2H.sub.2 .fwdarw.Ti+4HCl.DELTA.G=393.3 kJ/mol (1)
To cope with such an inconvenience, there is proposed a process of forming a Ti film by plasma CVD in which H.sub.2 is dissociated in a plasma and a metal halide such as TiCl.sub.4 is reduced by H atoms or activated species of H. The reaction is an exothermic reaction expressed by the following chemical equation (2): EQU TiCl.sub.4 +4H.fwdarw.Ti+4HCl.DELTA.G=-478.6 kJ/mol (2)
In this process, since the reaction easily proceeds as shown in the chemical equation (2), a Ti film can be formed at a relatively low temperature by plasma CVD. In particular, in this process, plasma CVD using a high density plasma source such as an ECR (Electron Cyclotron Resonance) plasma, ICP (Inductively Coupled Plasma) or helicon wave plasma is advantageous in terms of formation rate and uniformity of the film.
In plasma CVD using such a high density plasma source, however, there may occur an inconvenient state in which TiCl.sub.4 is not sufficiently reduced by H atoms or activated species of H in some film formation conditions. In this case, a nucleus density exerting an effect on growth of the Ti film is small, and thereby the Ti film grows non-uniformly, resulting in a granular homology of the film.
Also, in the case where TiCl.sub.4 is not sufficiently reduced, a lot of non-reacted TiCl.sub.4 or subchlorides of Ti, that is, TiCl.sub.x (x is a natural number selected from 1, 2 and 3) remain in the plasma. In such a case, a underlaying conductive material layer such as a silicon substrate is etched in accordance with the following chemical equation (3). EQU TiCl.sub.4 +3Si.fwdarw.TiSi.sub.2 +4SiCl.sub.4 .DELTA.G=-32.2 kJ/mol (3)
Such corrosion of the silicon substrate may break a shallow impurity diffusion layer on the surface of the silicon substrate, to thereby cause a serious defect such as the increased leak current.
Further, in the case where reduction of TiCl.sub.4 is imperfect, TiCl.sub.x or chlorine atoms are entrapped in a Ti film formed, so that the amount of chlorine remaining in the Ti film is increased. As a result, an Al based metal layer formed at the subsequent step may be corroded, and accordingly the contact resistance of the layer is increased, and in the worst case, there occurs disconnection of the layer.
Another problem is that unreacted TiCl.sub.4 or TiCl.sub.x acts as dust in a chamber. To be more specific, unreacted TiCl.sub.4 or TiCl.sub.x tends to be easily stuck and deposited on an inner wall of a CVD chamber. The deposits of unreacted TiCl.sub.4 or TiCl.sub.x, which contain chlorine in a large amount, are poor in adhesion with the inner wall, so that the deposits are simply peeled therefrom by thermal change due to turn-on/off of generation of plasma, to thereby deteriorate the particle level in the chamber or on the substrate to be processed.
Besides, various examinations have been made on formation of a TiN film by CVD such as thermal CVD, plasma CVD or MOCVD.
Of these plasma CVD processes, the thermal CVD process adopts a mixed gas reaction system between TiCl.sub.4 and NH.sub.3. This method has a merit in enhancing step coverage; however, when being applied to a substrate having a fine contact hole, it is disadvantageous in that a film is formed in a conformal shape both on a bottom surface and a side surface of the contact hole, and thereby an opening width of the contact hole is made narrow. This makes it difficult to fill the contact hole with tungsten by CVD or the like at the subsequent step. Specifically, in this case, there may occur a problem that a void is formed in the contact hole filled with tungsten, to thereby increase the contact resistance and reduce the reliability.
Further, in the mixed gas reaction system between TiCl.sub.4 and NH.sub.3, solid particles of ammonium chloride are produced as a reaction sub-product, which also deteriorate the particle level in the chamber or on the substrate to be processed.
For this reason, there has been proposed a method of burring a contact hole with a TiN film itself by thermal CVD; however, since the resistivity of TiN is about 10-30 times larger than that of tungsten, and consequently there may also occur a problem that the contact resistance is increased.
In plasma CVD, particularly, in plasma CVD using a high density plasma source for forming a TiN film, neutral species of TiN are produced by a mixed gas reaction system among TiCl.sub.4, N.sub.2 and H.sub.2, which allows effective film formation at a temperature of 500.degree. C. or less.
In a plasma CVD process for forming a Ti film or TiN film of a type in which incidence of species for film formation on a substrate to be processed is controlled by a divergent magnetic field, the incident direction cannot be sufficiently controlled because neutral species of TiCl.sub.x or TiCl.sub.4 are electrically neutral. In other words, at a peripheral portion of the substrate to be processed, the obliquely incident direction of the species toward the outer periphery of the substrate to be processed cannot be corrected. As a result, at a stepped portion of the peripheral portion of the substrate to be processed, for example, at a bottom portion and a side wall of a contact hole, the film is formed in an asymmetric shape in which one side is thin and the other side is thick. Such an asymmetry of coverage of the film becomes particularly inconvenient for a large diameter substrate of 8 inch or more.
To improve the asymmetry of coverage, there has been proposed a method of providing magnet coils for producing a mirror magnetic field under a substrate to be processed, thereby converging the flow of plasma at a peripheral portion of the substrate to be processed. The plasma CVD using such an electromagnetic configuration controls the incident direction of electrically charged species for film formation, and thereby it exhibits a certain effect on improvement in symmetry of coverage; however, such a plasma CVD fails to control the incident direction of neutral species for film formation.
Referring to FIGS. 12A and 12B, there will be described a problem resulting from the asymmetry of coverage of a TiN film indicated by reference numeral 51 in the figures.
FIGS. 12A and 12B show one side of an opening portion of a contact hole 3 formed in an interlayer insulating film 2 at a peripheral edge portion of a substrate to be processed, wherein the left side in the figures is equivalent to the center direction of the substrate to be processed. Neutral species of TiN flying obliquely from the center direction of the substrate to be processed allows a TiN film 51 to be grown in an overhang shape in the opening portion of the contact hole on the incident side of the neutral species of TiN. As a result, a necking portion 6 at which the thickness of the TiN film 51 is extremely small is formed directly under the overhang portion (see FIG. 12A).
Thus, upon formation of a W film 7 by CVD at the subsequent step, WF.sub.6 as a source gas passes through the thin TiN film 51 at the neck portion 6 and reaches the Ti film 5, so that TiF.sub.x is produced by reaction between WF.sub.6 and Ti film 5 and finally a mixture 8 of TiF.sub.x and W is produced. The mixture 8 of TiF.sub.x and W is weak in adhesive strength, and thereby the TiN film 51 is floated and peeled as shown in FIG. 12B. The peeling of the TiN film 51 not only reduces the reliability of a multi-layered interconnection but also becomes a cause of dust.